The present invention relates to a contact image sensor which uses an optical fiber array glass plate.
Contact image sensors are widely used in an image detection part of a facsimile and the like for reading the image of a paper. In general, such a contact image sensor as stated above must be equipped with a photoelectric conversion array, a detection circuit array and a light emitting diode (hereinafter referred to as LED) array, being formed of a structure in which one line of a paper is scanned under electrical control.
FIG. 1 is a cross-sectional view of a conventional contact image sensor equipped with a self focus micro lens array. A photoelectric conversion part 4 and a detection circuit 5 are formed on the insulating substrate 6 installed in the part of a protective plate 7, and a light emitting part 8 is attached to an inclined plane of the protective plate 7 and a self focus micro lens 3 is positioned vertically over the photoelectric conversion part 4 in the protective plate 7. An upper surface of a wear-resistant glass plate 2 is made to be positioned on the focal distance of the self focus micro lens 3, and a surface of the paper 1 is arranged to come into contact with the upper surface of the wear-resistant glass plate 2.
The photoelectric conversion part 4 converts a incident light into an electrical signal. Usually, the photoelectric conversion part 4 is composed of a photocell and a photodiode. The detection circuit 5 converts the output from the photoelectric conversion part 4 into a digital value.
The light which is irradiated from the light emitting part 8 to the direction of the paper permeates the transparent wear-resistant glass plate 2 and reaches the surface of the paper 1. The light reflected from the surface of the paper 1 is condensed in the photoelectric conversion part 4 through the self focus micro lens 3. The light condensed by the self focus micro lens 3 is converted into an electrical signal by the photoelectric conversion part 4. The detection circuit 5 converts the electrical signal into a digital signal, which is outputted to the outside. Here, the self focus micro lens 3 has a refractive index distribution like a formula (l) given below. ##EQU1##
The light incident upon the self focus micro lens 3 propagates forming a meandering curve according to the refractive index distribution like the formula (1), and the light incident is condensed to the photoelectric conversion part 4.
In the above formula (1), n(r) is the refractive index distribution in the direction of the radius of the self focus micro lens, n.sub.0 is a refractive index on the central axis of the self focus micro lens, A is the distribution constant of a refractive index and r is a distance in the direction of radius from the center.
However, such a contact image sensor as formed like FIG. 1 must use a self focus micro lens. According to the technical data CAT SLA Vol. 3 (October, 1987) published into Japanese, the optical dimension of practical self focus micro lens on the basis of the self focusing conditions used in a facsimile must represent that a view angle used in a facsimile is 20.degree. and that the length z.sub.0 of lens whose a distance between the paper and the lens is 17-18mm is about 8mm, as shown in FIG. 2. Thus, it is difficult to miniaturize the contact image sensor to be formed as shown in FIG. 1. Moreover, the self focus micro lens 3 is complicated in manufacturing process because the lens has a particular refractive index distribution. It also involves a problem in that the lens is a very expensive one among the constituent parts of the sensor.
FIG. 3 is a cross-sectional view of another conventional contact image sensor. A photoelectric conversion part 4 with an opening 13 formed in its center for passage of light is formed of thin film on the transparent glass plate 12 and the circumference of the opening 13 is so made as not to pass the rays of light by covering the lower part of photoelectric conversion part 4 with a thin metallic film made up of a metallic conduction layer. The upper part of photoelectric conversion part 4 manufactured in such a manner is coated with transparent resin and a wear-resistant protective glass plate 10 is stuck thereto. The light emitting part 8 is attached to the protective plate 7 positioned in the lower part of transparent glass plate 12 and is formed to irradiate rays of light. The paper 1 is in contact with the upper surface of wear-resistant protective glass plate 10.
Referring to an operations of the contact image sensor formed like the FIG. 3, it is as follows. The light irradiated from the light emitting part 8 within the protective plate 7 permeates the transparent glass plate 12, passes through the opening 13 of the photoelectric conversion part 4 and permeates the transparent resin 11 and the wear-resistant protective glass plate 10 and reaches the paper 1. The light reflected from the paper 1 is made incident upon the photoelectric conversion part by permeating again the wear-resistant protective glass plate 10 and the transparent resin 11. At this time, the photoelectric conversion part 4 converts the quantity of incident light which is reflected from the paper 1 into an electrical signal and inputs the electrical signal to the detection circuit 5. The detection circuit 5 digitalizes the inputted signal and outputs the digitalized signal to the outside.
In the case where it is formed as shown FIG. 3, the self focus micro lens 3 is not used as a image focus lens, but the transparent resin 11 and the wear-resistant protective glass plate 10 are used in the upper of the photoelectric conversion part 4. However, the light reflected from the surface of paper 1 is weakened and dispersed while passing through the wear-resistant protective glass plate 10 and the transparent resin 11 and so a light route must be minimized by thinning the thickness of photoelectric conversion part 4. Thus, it is difficult to assemble a contact image sensor. When a resolving power rises, it is difficult to resolve an image and the output characteristic of the sensor tends to be thereby weakened. In the case where the wear-resistant protective glass plate 10 is scratched, the function of the sensor deteriorates.